The determinants critical for the incorporation of Pr160gag-pol into HIV-1 particles were examined by cotransfecting cells with: 1) a plasmid expressing wild type Gag protein and 2) a series of chimeric Gag-Pol expression plasmids in which individual murine leukemia virus (MLV) Gag regions/subdomains precisely replaced their HIV-1 counterparts. The presence of MLV MA and NC Gag regions in the chimeric Gag-Pol precursor had no detectable effect on its incorporation into progeny virions. In contrast, the entire HIV-1 CA region was required to achieve wildtype levels of Gag-Pol assembly into particles; the CA major homology region (MHR) and adjacent C-terminal CA sequences appeared to play dominant roles in this process yet, when assayed in the context of a chimeric Gag-Pol polyprotein, only partially restored the defect in Gag-Pol incorporation. The two principal cell targets for HIV-1 infection in vivo are CD4+ T lymphocytes and tissue macrophage. HIV-1 encodes envelope glycoproteins which mediate virus entry into these cells. Infected and radiolabelled primary peripheral blood mononuclear cell (PBMC) and monocyte-derived macrophage (MDM) cultures were utilized to examine the biochemical and antigenic properties of the HIV-1 envelope produced in these two cell types. The gp120 produced in MDM migrates as a broad, diffuse band in polyacrylamide gel electrophoreses compared with that of the more homogenous gp120 released from PBMCs. Glycosidase analyses indicated that the diffuse appearance of the MDM gp120 is due to the presence of asparagine-linked carbohydrates containing lactosaminoglycans, a modification not observed with the gp120 produced in PBMCs. Virus neutralization experiments, using isogenic PBMC and MDM-derived macrophage-tropic HIV-1 isolates, revealed that 8 to 10-fold more neutralizing antibody, directed against the viral envelope, is required to block virus produced from MDM. These results indicate that HIV-1 released from infected PBMC and MDM cultures differs in its biochemical and antigenic properties.